The RSPCA, RSPB, Devon, Cornwall and Dorset wildlife trusts and the British Trust for Ornithology said an estimated 2,400 birds had died and more than 500 live but "disabled" birds had been rescued and taken for treatment.

The true death toll is thought to be even higher, they said.

It said PIB becomes strongly adhesive in the sea, coating the birds and restricting their movements and their ability to feed.

Tony Whitehead, from the RSPB, said: "This is one of the worst marine pollution incidents in decades, bringing to mind other disasters going right back to the Torrey Canyon in 1967."

Adam Grogan, from the RSPCA, said: "The dumping at sea of this lethal chemical must be stopped.

"It was heartbreaking enough after the first incident in February to see so many birds arrive at our centres in such a poor state.

"The sticky substance coated their feathers and made it difficult for them to feed and move so staff had to work around the clock to wash it off and get them fit enough to survive in the wild again.

"For it to happen twice in quick succession is inexcusable and unacceptable."

The charities have urged members of the public to sign petitions by Avaaz and 38 Degrees to support their call for a ban of dumping chemicals at sea.

Post-mortem examinations have been carried out on hundreds of the dead birds by the British Trust for Ornithology and the results are due to be published within the next few weeks.